The major goal of this study is to implement innovative, noninvasive imaging techniques to study the mechanisms involved in the loss and recovery of muscle strength and overall functional ability after limb disuse. We will focus our investigation on a number of intrinsic properties of skeletal muscle. Particular attention will be focused on the muscle contractile area, muscle specific force, and the inorganic phosphate concentration. For this purpose, Magnetic Resonance Imaging and Magnetic Resonance Spectroscopy measurements in combination with force measurements will be performed on skeletal muscle during the course of cast immobilization and the subsequent recovery, in both patients and animals. The loss of muscle strength with limb disuse has been related to a decrease in muscle contractile area as well as a decrease in muscle specific force (force/contractile area). We hypothesize that an increase in the basal inorganic phosphate (Pi) content contributes to the decline in muscle specific force after limb disuse. Conversely, we hypothesize that the recovery of muscle specific force is associated with a decrease in basal inorganic phosphate content. This hypothesis is tested using two approaches: a) We will determine the relationship between muscle specific force and in vivo basal inorganic phosphate content during the course of cast immobilization, reloading and the subsequent recovery and b) using skinned muscle fibers we will determine the direct effect of the inorganic phosphate concentration on the force output per fiber diameter. Skinned fiber studies will be performed on experimental muscles (casted, reloaded and recovered) at control Pi concentrations and elevated Pi concentrations, as determined in vivo by 31P-spectroscopy. However the ultimate goal of this study is to determine how in vivo determined pathophysiological changes affect the overall functional ability. For this purpose, the ability to perform simple functional tasks such as stairclimbing, walking and one legged heel rises will be assessed during the experimental timecourse. We anticipate that the investigation of muscle dysfunction from a cellular to a whole body level as proposed in this study will help provide the understanding necessary for the development of preventive and therapeutic interventions designed to minimize the loss of functional ability that accompanies limb disuse.